As punishment for a prank, Bart is sentenced to assist Principal Skinner with his astronomy project, which begins at 4:30 each morning. While on detail with Skinner, Bart spots a comet and reports it to an observatory. The astronomers at the observatory congratulate Bart for his discovery and name the comet after him. Skinner is crestfallen.

It becomes apparent that "The Bart Simpson Comet" is on a collision course with Springfield. Scientists launch a rocket to intercept the comet and blow it to pieces, but the rocket misses its target and destroys the bridge that is the only way out of town.

The residents of Springfield take refuge in Flanders's bomb shelter. However, not everyone can fit in the tiny space. Homer forces Flanders out of the shelter, but, feeling horribly guilty, he opens the shelter door. Homer and the others step outside. As the comet enters the atmosphere, it enters a thick layer of pollution and begins to break up until it seems harmlessly amall. The comet lands on Flanders's empty shelter, causing it to collapse instantly.